A device designed to detect the presence and determine the concentration of oxygen (O2) by paramagnetic, electrochemical, or other technique. The device is typically used as a component of an oxygen monitor or a patient monitoring device/system that determines the percentage of O2 in medical gases used in an enclosed atmosphere (e.g., an anaesthesia or ventilator circuit, oxygen tent, oxygen therapy device/system tubing) inspired by a patient. This is a reusable device.